The present invention relates to bearings for rotary machines and, specifically, to a tilt pad journal bearing assembly having reduced width bearing pads.
Tilt pad bearings are commonly used in large, high speed, rotating machinery such as gas and steam turbines and high speed compressors. For example, tilt pad bearings used for a turbine rotor provide the only practical means by which vibrations in the journal may be damped. Although most applications of the tilt pad bearing are generally successful, it is recognized that, in some situations, the tilt pad bearing""s capacity for vibration damping is less than desired.
Typically, the bearing assembly is arranged concentrically about the journal of, for example, a compressor rotor, and include a plurality of arc segments, or tilt pads that are retained within a casing that is also concentric with the journal. Each tilt pad has a pad body with an inner bearing or journal engaging surface (or, simply, journal surface) which is closely concentric to the cylindrical surface of the journal, and an outer surface that includes a pad mounting component that allows the pad to self-center on the journal. The journal surface itself is continuous and smooth over the entirety thereof.
Problems have been experienced with bearing pads of the above described type in certain high speed compressors. Specifically, a problem with synchronous instability known in the literature as the xe2x80x9cMorton Effectxe2x80x9d has been experienced in high speed single stage compressors running at approximately 10,500 rpm. This so-called Morton Effect is characterized by continuously rising vibrations at constant (for example, rated) speed.
In the exemplary embodiment of this invention, the width of each tilt pad journal surface is reduced from either side by relieving the marginal edges of the pad to form marginal undercut surfaces or lands. The journal surface width may thus be reduced by about 50%, thereby increasing the unit loading on the journal surface. It was discovered unexpectedly that this modification also reduces vibration and eliminates the synchronous instability problem in certain high speed compressors. In the exemplary embodiment, the extent of the relief or undercut is approximately 0.050 inch.
Accordingly, in one aspect, the present invention relates to a bearing pad for a tilt pad journal bearing assembly comprises a pad body having a concave journal surface on one side and a mounting portion on an opposite side adapted to secure the pad to a bearing casing, the journal surface having undercut surfaces extending along opposite marginal edges thereof.
In another aspect, the invention relates to a bearing pad for a tilt pad journal bearing assembly comprises a pad body having a concave journal surface on one side and a mounting portion on an opposite side adapted to secure the pad to a bearing casing, the journal surface having undercut surfaces extending along opposite marginal edges thereof.
In still another embodiment, the invention relates to a method of reducing vibration in a tilt pad journal bearing assembly having a casing and a plurality of bearing pads mounted in the casing, the method comprising a) providing individual arcuate tilt pads having concave journal surfaces; and b) forming relieved surfaces on either side of the journal surface along marginal edges thereof in an arcuate length dimension.
The invention will now be described in greater detail in conjunction with the following drawing figures.